The field of biomaterials has become an area of vital importance because these materials increase the quality and longevity of human life and the science and technology associated with this field has led to the generation of a billion-dollar business. Biomaterials are polymers, metals, ceramics, inorganic materials, and synthetic and natural macromolecules (biopolymers), which are manufactured or processed to be used within or as medical devices or prostheses. These materials typically come into contact with cells, proteins, tissues, organs, and organic systems. The shape of the material and how it interacts with the environment (in blood contact for example) and its time of use will determine the properties required, suitable for a specific application.
A biomaterial must have as fundamental properties: a) good mechanical properties like hardness, tensile strength, modulus and elongation, from which its specific application is established; b) biocompatible, that is, it must be of low toxicity and not generate inflammatory or allergic reactions in the host, hence, the two relevant factors that influence a material's biocompatibility are the host response and the material degradation in physical environments; c) high corrosion and wear resistance understood as low wear and corrosion resistance of the implants in bodily fluids, given that wear results in the release of incompatible metal ions that cause allergic and toxic reactions in the host; d) osseointegration, that is, the incapacity of a surface on an implant of integrating with the adjacent bone and other tissues due to micro-movements, which result in loss of the implant. Thereby, materials with a suitable surface are essential in the integration process of an implant with the bone or the surrounding tissue. Surface chemistry, roughness, and topography play a dominant role in the development of good osseointegration.
Materials currently used for surgical implants include 316L stainless steel (316L SS), cobalt-chromium alloys (Co—Cr), and titanium alloys (Ti). Elements like nickel (Ni), chromium (Cr), and Co can be released from steel and from Co—Cr alloys due to corrosion in the physical environment. The toxic effects of metals (Ni, Cr, Co) released from prostheses implants have been studied and dermatitis has been reported due to the toxicity of the Ni element and numerous “in-vivo” studies in animals have shown carcinogenicity due to the presence of Co. These materials (316L SS and Co—Cr alloys) have a higher modulus of elasticity than that of bone (30-40 GPa), which causes insufficient capacity to transfer stress, resulting in bone resorption and weakening of the implant after some years of implantation. Commercially, pure Ti and the Ti-6Al-4V ELI alloy (Ti64-Extra Low Interstitial) are the most commonly used materials for applications in implants with low modulus of elasticity values that vary from 110 to 55 GPa compared to 316L SS (210 GPa) and Co—Cr alloys (240 GPa). Although titanium and its alloys, especially Ti64, have excellent corrosion resistance and are biocompatible, their long-term performance has raised concern due to the release of aluminum (Al) and vanadium (V) ions, which have been associated with health problems like Alzheimer's disease, neuropathy, and bone diseases like osteomalacia. Besides, V is toxic in its elemental form, as well as in oxide (V2O5) form, present on the surface. Titanium has low resistance to cutting, which makes it less applicable to the manufacture of screws for bones, plates to join bones, and similar applications. Finally, titanium and its alloys show severe wear when in contact with other metals, in addition to having a high friction coefficient, which leads to the formation of wear particles (debris), resulting in inflammatory reactions causing pain and softening of implants due to osteolysis. Given the limitations mentioned, the service period of implants manufactured with these materials is restricted to 10-15 years. This has stimulated research on developing new materials and processes to modify these surfaces.
To modify the surface of a substrate of biomedical application, the physical properties of the base material must be taken into account in order for the PVD-sputtering process to be highly reproducible, so that the surface properties of the protective coating are those sought for the biological application required. Properties like: roughness, friction, wear, hardness, modulus of elasticity, chemical inertness, corrosion resistance, and biocompatibility are essential when generating thin layers of nitrides, carbides, and oxides as protective systems of biocompatible substrates.
Keeping these facts in mind, there is a wide range of materials that fulfill this function, given their tribological or mechanical properties, in a way that positive response is integrally shown between the material to be implanted and the tissue that will host it. An example of this are urinary catheters, which, with the tissue, form incrustation-type bio-coatings where high bacterial proliferation exists, generating physiological complications; this can be avoided, as shown by research conducted by N. Laube, using a surface treatment with diamond-like carbon (DLC) [1,2]. Urinary catheters are not the only ones with this type of problem, devices like heart valves, coronary stents, and capillary tubes may generate hemostasis, produced by direct contact between the biomaterial and blood flow, where protein absorption takes place, leading to the adhesion and activation of platelets [3]; the blood flowing produces a shear effect on the surface of the biomaterial, affecting the tissue-coating system; these are some examples where the surface modification, whether through diminished friction coefficient, surface roughness, or chemical inertness may solve drawbacks.
Studies carried out by E. De Las Heras and F. Walthera [4] show that the type of surface treatment must be chosen adequately. One of the most currently used treatments is the nitriding procedure; nitriding 316L steel through plasma-assisted techniques results in increased surface hardness accompanied by diminished anticorrosion properties; nitriding extended austenitic phases in the protection process are generated accompanied by nitrides Cr2N and Fe4N [5], which correspond to a special triclinic arrangement [6]. To avoid these adverse biological effects, metallic and ceramic materials are used, which can be used as surface treatment to improve mechanical, tribological, anticorrosion, and biological properties. Among the metallic materials, there are Ti and zirconium (Zr) and ceramics like the nitrides of some transition metals (TiN, ZrN), carbides (TiC, SiC), oxides like TiO2, Al2O3, and ZrO2 and diamond-like carbon (DLC); these synthesized materials are employed as surface treatment using the PVD-sputtering technique [7, 8, 9, 10] and, normally have a thickness in the order of one to two micrometers. Multilayers have been used with good results since the 1970s, achieving better properties when compared to monolayers; the multilayered structure acts as a cracking inhibitor and, consequently, the coating's fracture toughness increases; multilayers normally use binary compounds, for example, TiN/TiC, TiN/ZrN and some have used their generic materials like: W/WC, Ti/TiN, c-BN/TiN [11, 12, 13, 14]. Multilayered coatings also have high hardness and excellent corrosion resistance.
When seeking to implant an inorganic biomaterial, such inorganic biomaterial has an effect on cellular action, which recognizes environmental signals that alter its phenotype. The specific properties that stimulate cell action on the surface take place due to four types of surface-tissue interactions:                Topography: directly implies cellular and molecular adherence on the surface; surface roughness, variable to discern when studying cell fixation.        Surface energy: it has been demonstrated that the preferential crystallographic orientation of the protective coating also plays a preponderant role in cell activity as far as its organization and fixation; these facts have been reported by Chiung-Fang Huang and Faghihi for osteoblastic cells in Ti6Al4N alloys, in (100) and (110) directions, which reflects positive changes in biocompatibility [15, 16].        Self-diffusion of metallic cations: corrosion system, through high porosity and generation of mechanical wear problems producing particles. Many metals in pure state cause biocompatibility problems, for example: Ni (carcinogenic), 316L (release of Fe), Co, Cu, and V (highly toxic) [17, 18], hence, needing surface treatments, or passivation, to regulate the problem of anti-biocompatibility, bearing in mind that said treatments produce biocompatible materials soon after the implant; to cite some: TiAIV, Co—Cr—Mo, and Ti—Ni.        Tribological problems: metal ions and atomic groups or particles generated by wear processes cause adverse effects, which stimulates macrophages and T lymphocytes, which leads to cytokine production [19] that causes acute or chronic inflammation if the injurious agent persists [20].        
From the aforementioned, two problems have been established around including thin layers as surface treatment: adherence and porosity. Coatings like Ti and Zr are part of the so-called transition metals [21, 22],  which through reactive nitrogen gas sputtering form ZrN and TiN, which have similar physical and chemical characteristics, making them candidates as biocompatible material. Many studies have been conducted on surface modification based on these nitrides, for example, Hontsu et al. [23] demonstrated the pertinence of generating a TiN surface treatment for AISI 317L steel substrates and dope it with silver (Ag) particles provoking an antibacterial surface treatment.
One of the problems presented by 317L stainless steel is that it does not fulfill antibacterial functions, but when doped with Ag it acquires diminished anticorrosion properties; applying TiN coatings enables synergy with Ag ions [23]. Other thin-layered coatings exist, in ternary form like titanium aluminum nitride (TiAlN) or TiN/TiAlN multilayers, which are also part of the possible surface treatments of biocompatible substrates. Professor Braic from the Institute for Chemical-Pharmaceutical Research and Development in Romania [24] studied the corrosion and biocompatibility of TiN and TiAlN monolayers, in addition to (TiN/TiAlN)n multilayered coatings, with n number of bilayers, all using continuous arc PVD techniques, comparing the corrosion levels with respect to an alloy that is currently highly used for hip implants, Co—Cr—Mo. Regarding its biocompatibility results, the best viability was found for TiAlN coatings, followed by (TiN/TiAlN)720 and TiN. As a result of the biological tests, all the coatings studied showed good biocompatibility, without sign of cytotoxicity [24]. One of the surface treatments used most frequently are the nanometric layers of DLC or amorphous carbon (a-C); chemically, when atoms are not in their graphitic or diamond-like phase, their bond form is sp1 type (two valence electrons forming σbonds and the others in py and pz orbitals, forming πbonds), sp2 or sp3, the material obtained has different properties [25]. In DLC structures, bonds are normally sp2 and sp3 type, in different concentrations with hardness in the order of 30 GPa, transparent and with high wear resistance [26], making it a high-performance material, besides being biocompatible, by being chemically inert. Another carbon-based coating is the amorphous carbon nitride (a-CNx ), used for cutting and machining tools and because of its biological inertness as biomaterial, besides having excellent antiwear properties and low friction coefficient. This material is normally synthesized via PVD RF sputtering, through reactive methods, with CH4, N2, and argon [7].
In patents literature, reports are found for materials used as coating in multilayers. The closest anticipations to the invention correspond to some documents divulged on the state-of-the-art, like patent EP0366289 (MIDWEST RESEARCH TECHNOLOGIES), which shows as modalities of the invention multiple alternating layers of: (1) metal (Ti, Zr, Hf, Ta) and ceramic material (a metal nitride), (2) metal (Al, Si, Ti, Cr, Mg, Fe, Zr, Mo, W, Ta) and ceramic material (a metal oxide), (3) metal (Ti, Zr, Hf, Fe, or Ta), and ceramic material (a metal carbide), where each of the layers has a thickness of 0.1 to 5 μm and the substrate is a metal with a 2-μm thickness.
Patent request WO2007136777 shows a coating for an electricity conducting substrate made up of a metallic layer (Ti, Cr, Va, Al, Mo, Nb, W, Hf, Zi or alloys of such) followed by a ceramic layer (nitrides, carbides, oxides, oxicarbides, oxinitrides, borides, carboborides, borocarbonitrides, and combinations), and a diamond-like amorphous matrix (C, Si, N, H, O or transition metals) with a friction coefficient below 0.3, where the metallic and ceramic layer present a thickness of 0.01-30 μm and the amorphous matrix a thickness between 0.01 and 30 μm.
Patent request WO2010086598 divulges a coating for a substrate that comprises a layer of Ti, Cr, or an alloy of such, a metallic layer (NiTi, Ni, Ti, Cr, Al, Pt, Hf, Zr, Co, Cu, or Y) and a ceramic layer (Al or Si or a combination of nitrides, carbides, oxides, or borides of metals from groups 4, 5, or 6), which includes at least four layers of metal and ceramic material, where the coating has a thickness between 0.1 and 5 μm.
U.S. Pat. No. 4,904,542 shows a multilayer-type coating resistant to form a metallic substrate that comprises a metallic layer (Ti, Hf, Zr, Ta), a ceramic layer (metal nitride) that includes at least four metal and ceramic layers with thickness between 0.1 and 5 μm, preferably a 2-μm metallic layer and the ceramic layer with thickness of 0.5 μm. Likewise, it divulges an additional modality that comprises a metallic layer (Al, Si, Ti, Cr, Mg, Fe, Zr, Mo, W, Ta) and a ceramic material selected from a metal oxide (Al, Si, Ti, Cr, Mg, Fe, Zr, Mo, W, Ta). Another modality specifies a multilayer coating made up of a metallic layer (Ti, Hf, Zr, Fe, Ta) and a layer of ceramic material selected from a metal carbide (Ti, Hf, Zr, Fe, Ta).
In spite of the different materials existing in the state of the technique, there is need for a coating with improved mechanical and biological properties for substrates used in surgical implants (316L SS, Co—Cr alloy, Ti alloy—like Ti 64 and/or other materials). The coating of the invention solves the shortcomings of existing materials by disclosing a multilayered thin-film coating (S/TiN/Ti/TiZr) for substrates used in surgical materials that offers corrosion and wear resistance of said material with low genotoxicity, cytotoxicity and without effects in osseointegration.